1. Field
The following description relates to an insulation material composition for direct current (DC) power cables, and, more particularly, to an insulation formed from an insulation material composition that may be used to DC power cables which are suitable as high voltage DC transmission lines.
2. Description of Related Art
A power cable being currently used includes a conductor 1, an inner semiconducting layer 2, an insulation 3, an outer semiconducting layer 4, a lead sheath 5 and a polyethylene (PE) sheath 6, as shown in FIGS. 1a and 1b. 
Polyethylene and crosslinked polyethylene have been widely used as the insulation 3 of the power cable. However, in a case where a power cable with an insulation made from polyethylene or crosslinked polyethylene is used as a high voltage transmission line, several problems may occur. For example, when a high voltage DC is applied to a cable, a space charge may generate due to movement of electric charges from an electrode into an insulation and influences of crosslinking by-products. If such a space charge is accumulated in a polyethylene insulation by a high voltage DC applied to a power cable, the electric field strength of the power cable near a conductor increases, resulting in reduced breakdown voltage of the cable.
To solve the problem, solutions have been suggested to form an insulation using magnesium oxide. Magnesium oxide basically has a face centered cubic (FCC) crystal structure, but it may have various shapes, purity, crystallinity and properties according to synthesis methods. Magnesium oxide has cubic, terrace, rod-like, porous and spherical shapes, as shown in FIGS. 2a through 2e, and each shape may be used depending on specific properties. In particular, spherical magnesium oxide is used to suppress a space charge of a power cable, as suggested in Japanese Patent Nos. 2541034 and 3430875. As mentioned above, studies have been made to suppress a space charge in a power cable with an insulation made from polyethylene or crosslinked polyethylene. Therefore, an insulation material composition for a DC power cable having an excellent space charge suppression effect is desired.